Handsfree or loudspeaking telephone instruments using amplitude controlled switching of the voice channels are, of course, well known. In such circuits, a voice signal generated locally for transmission is compared against a voice signal being received for amplitude and the path for the louder signal is switched on while the lesser signal path is attenuated. The comparison continues and the circuit switches to pass the lesser signal when in fact it becomes the louder signal. In one form of this circuit, as shown in U.S. Pat. No. 3,725,585 issued to Moniak et al. Apr. 3, 1973, an incremental signal is added to the louder signal to provide a higher threshold which must be overcome by the then lesser signal to cause a switch over.
When a handsfree or loudspeaking instrument feature is to be applied to a telephone instrument of the type shown by U.S. Pat. No. 4,315,110 issued to J. Davis Feb. 9, 1982, provision must be made for transforming hookswitch signals in data form into operative signals for handsfree operation.
Further, electronic subsets using dynamic transducers employ electronic speech or voice networks which may be of the type shown by U.S. Pat. No. 4,286,123 issued to P. Blomley Aug. 25, 1981. Such voice networks may provide interfacing with the telephone dial especially a dial of the DTMF (dual tone multi-frequency) type to enable the DTMF signals to pass through the speech network to the lines.
A feature which has become popular in PBX systems with current electromagnetic and electronic subsets is that of call announcing. With this feature, a loudspeaker at stations so equipped is accessed for a timed period to allow a message of limited duration to be broadcast by the speaker when the called station is on-hook. A variation on this feature, two-way call announcing, allows the called station to respond through a microphone associated with the instrument.